WO02/074548, WO06/018640, WO07/045912 teach paper that has been surface coated with a light activated colour change technology. The light activated colour change technology is firstly formulated into a liquid ink together with a binder and compatible liquid carrier. The liquid ink is then coated onto the paper using a printing technique such as flexography. Drying the substrate then removes the carrier to leave the light activated colour change technology bound to the surface of the paper by the binder. This technique has several drawbacks such as the use of a wasteful and time consuming printing stage.
U.S. Pat. No. 6,306,493 teaches paper and board products comprising micronized polymers such as linear aromatic polyesters and/or linear polyarylenes as the absorber material and the material for carbonization. This method however, only produces grey-scale brown-burn marks.